Method and device for optical switching and variable optical attenuation

ABSTRACT

An optical switching device for switching M input signals into N output signals, wherein M and N are each equal to or greater than one, including a mode division multiplexer to join the M input signals into a first multi-mode signal having M initial modes, a mode converter to convert the first multi-mode signal into a second multi-mode signal having N converted modes, and a mode division demultiplexer to separate the second multi-mode signal into the N output signals, wherein the convener is able to be controllably activated such that the N converted modes are separated by the demultiplexer to the N output signals according to a desired scheme.

FIELD OF THE INVENTION

The invention relates to the switching of optical signals betweenalternate waveguides, such as optical fibers

BACKGROUND OF THE INVENTION

In the field of integrated optics, there may be a need to use a switch,e.g., to route signals and/or to add or subtract signal-carrying lines.

U.S. Pat. No. 5,915,050 to P. Russell, et al describes an optical deviceincluding an optical fiber directional coupler supporting at least twopossible electromagnetic transmission modes. An acousto-optic effect isused to create a spatial periodic perturbation, which allows powertransfer between the transmission modes.

U.S. Pat. No. 5,418,868 to Cohen et al describes a thermally activeoptical device based on a Mach Zehnder Interferometer (MZI) opticallycoupled to an adiabatic 3 dB input coupler and an adiabatic 3 dB outputcoupler. One arm of the MZI includes a thermo-optical phase shifter. Thethermo-optical phase shifter may be used to control the distribution oftransmitted light at the output of the device.

A dynamic device is defined as a device in which an optical property ofthe device may be altered, e.g., the device may be turned on/off and/ora refractive index of the device may be changed. A passive device isdefined as a device in which all optical properties are pre-determinedand depend only on a power distribution of a light injected into thedevice.

SUMMARY OF THE INVENTION

According to embodiments of the invention, an optical switch is providedfor switching M input signals into N output signals, wherein M and N areequal to or greater than one. The switch may include a Mode DivisionDemultiplexer (MDM) to join the M input signals into a first multi-mode(MM) signal having M initial modes. The switch may additionally includea Mode Converter (MC) to convert the first MM signal into a second MMsignal having N converted modes. The switch may also include a ModeDivision Demultiplexer (MDD) to separate the second MM signal into the Noutput signals. According to embodiments of the invention, the MC may becontrollably activated such that the N converted modes are separated bythe demultiplexer into the N output signals according to a desiredscheme.

According to embodiments of the invention, the switch may include an“off” state and at least one “on” state of operation.

According to exemplary embodiments of the invention, the MDM may have anoutput and M inputs associated with M single-mode (SM) input waveguides,respectively. The output of the MDM may be associated with an input ofthe MC by a first MM waveguide. An output of the MC may be associatedwith an input of the MDD by a second MM waveguide. The MDD may have Noutputs associated with N SM output waveguides.

According to some exemplary embodiments of the invention, the MC mayinclude a dynamic MC having a long period grating (LPG). In someembodiments, the dynamic MC may be selectively controlled to convert adesired fraction of a signal from a first guided mode to a second guidedmode.

According to additional exemplary embodiments of the invention, the MCmay include an adiabatic interferometer based MC.

According to other exemplary embodiments of the invention, the MC mayinclude a splitter-based MC.

Some exemplary embodiments of the invention provide a Variable OpticalAttenuator (VOA) implemented by a switch, for example, a switch asdescribed herein, associated with one input waveguide and one outputwaveguide. According some embodiments of the invention, the VOA may beimplemented as a “normally bright” VOA, wherein the switch transmitslight signals from the input waveguide to the output waveguidesubstantially un-attenuated when the switch is at the “of” state.According to other embodiments, the VOA may be implemented as a“normally dark” VOA, wherein the switch substantially blocks the lightsignal from being transmitted from the input waveguide to the outputwaveguide when at the “off” state.

According to some embodiments of the invention, the MC may include aplurality of cascaded sub-mode converters, wherein each mode convertercontrols switching between at least one different pair of modes

Some exemplary embodiments of the invention provide a 2×2 switch adaptedfor switching between two input signals and two output signals Accordingto these embodiments, M and N each equal two and the MC is adapted toconvert between two-order modes, e.g., a fundamental mode and afirst-order mode.

According to another embodiment of the invention, an optical 2×2 switchmay include two SM input waveguides, a Y coupling branch structure, afirst MM waveguide, a MDD, two SM MD waveguides, two controllable phaseshifters, e.g., heating elements, a MDM, a second MM waveguide, asplitting Y-branch structure, and two SM output waveguides.

According to a further embodiment of the invention, a method is providedfor switching M input signals into N output signals. The method mayinclude joining M input signals into a first MM signal having M initialmodes. The method may additionally include converting the first MMsignal into a second MM signal having N converted modes. The method mayfurther include separating the second MM signal into N output SMsignals.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanied drawings in which:

FIG. 1 is a schematic illustration of a planar structure of a 1×Noptical switch in accordance with some exemplary embodiments of thepresent invention;

FIG. 2A is a schematic illustration of a MM directional coupler that maybe used in conjunction with exemplary embodiments of the invention;

FIG. 2B is a schematic illustration of an adiabatic mode demultiplexerthat may be used in conjunction with exemplary embodiments of theinvention;

FIG. 2C is a schematic illustration of a long period grating modeconverter that may be used in conjunction with exemplary embodiments ofthe invention;

FIG. 2D is a schematic illustration of a splitter-based mode converterthat may be used in conjunction with exemplary embodiments of theinvention;

FIG. 3 is a schematic illustration of a planar structure of a M×Noptical switch in accordance with exemplary embodiments of the presentinvention;

FIG. 4 is a schematic illustration of a cascade of sub mode converters,in accordance with exemplary embodiments of the present invention;

FIG. 5 is a schematic illustration of a 2×2 “normally bar” opticalswitch configuration in accordance with exemplary embodiments of theinvention;

FIG. 6 is a schematic illustration of a 2×2 “normally cross” opticalswitch configuration in accordance with exemplary embodiments of theinvention;

FIG. 7 is a schematic illustration of a planar structure of another 2×2optical switch, in accordance with additional exemplary embodiments ofthe invention; and

FIG. 8 is a schematic block-diagram illustration of a method forswitching M input signals into N output signals in accordance withexemplary embodiments of the invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn accuratelyor to scale. For example, the dimensions of some of the elements may beexaggerated relative to other elements for clarity or several physicalcomponents included in one element. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements. It will be appreciatedthat these figures present examples of embodiments of the presentinvention and are not intended to limit the scope of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those of ordinary skill in the artthat the present invention may be practiced without these specificdetails. In other instances, well-known methods, procedures, componentsand circuits may not have been described in detail so as not to obscurethe present invention.

Reference is made to FIG. 1, which schematically illustrates a planarstructure of a 1×N optical switch 100 in accordance with some exemplaryembodiments of the present invention.

According to embodiments of the invention, 1×N optical switch 100 mayinclude a Single Mode (SM) input waveguide 102, a passive waveguideexpander 104, a first Multi-Mode (MM) waveguide 106, a dynamic Modeconverter (MC) 108, a second MM waveguide 110 and a passive ModeDivision Demultiplexer (MDD) 112 including N output waveguides 114,wherein N is equal to or greater than one.

According to exemplary embodiments of the invention, a SM light signalmay enter an input 118 of expander 104 via input waveguide 102. Passiveexpander 104 may convert the SM signal into an initial MM signal exitingexpander 104 at an output 120. The initial MM signal may propagate viaMM waveguide 106 and enter an input 122 of MC 108 Dynamic MC 108 may beused to convert the initial MM signal into a converted MM signal exitingMC 108 at an output 124. The converted MM signal may propagate via MMwaveguide 110 and enter an input 126 of MDD 112. Passive MDD 112 may beadapted to separate each mode of order j of the converted MM signal intoa corresponding output SM waveguide 114 supporting the mode of order j.According to embodiments of the invention, MC 108 may be selectivelycontrolled to convert the initial MM signal into the converted MM signalsuch that MDD 112 may separate each mode of order j into a desiredoutput waveguide 114. Thus, switching of the input signal to any one ofoutput waveguides 114 may be controlled by selectively controlling arelationship between the initial MM signal and the converted MM signal,using the MC 108, such that the converted signal includes the mode oforder j corresponding to the desired output 114 of MDD 112, as describedbelow.

According to embodiments of the invention, dynamic MC 108 may operate inconjunction with MM waveguides 106 and 110, respectively, and may haveat least two states of operation, e.g., at least one “On” state and an“Off” state. When the MC is at the “Off” state, a mode of order i entersthe MC and the same mode of order i exits the MC. When MC 108 isswitched to the “On” states, a mode of order i entering the MC is atleast partially converted into a mode of order j that exits the MC.

According to embodiments of the invention, optical switch 100 may haveat least two states of operation, e.g., at least one “On” state and an“Off” state, which may correspond, for example, to the states ofoperation of MC 108. When MC 108 is at the “off” state, a signalentering the switch from SM input waveguide 102 may exit the switch viaa predetermined output waveguide, e.g., waveguide 115, without beingaffected by the switch.

When the MC is switched to the “on” states, it may allow converting afundamental mode of the entering signal, into a certain high-order mode,e.g., a second-order mode, such that the MDD may separate the signal,according to a desired scheme, into a desired waveguide of outputwaveguides 114, e.g., waveguide 116.

According to an exemplary embodiment of the invention, switch 100 may bea 1×2 switch, in which case MC 108 may have one “on” state of operation,to switch between two optical modes, for example, a fundamental mode anda higher-order mode, e.g., a first-order mode. According to thisembodiment, the MDD may separate the fundamental mode into a firstpredetermined output waveguide, and the higher-order mode into a secondpredetermined output waveguide.

According to embodiments of the invention, passive waveguide expander104 may include any suitable waveguide expander known in the art, andmay transform a mode of order i received from a SM waveguide into acorresponding mode of a different shape, e.g., a single mode with thesame mode-order i, adapted to propagate in a MM waveguide.

According to exemplary embodiments of the invention, passive expander104 may be used to expand an initial mode of SM waveguide 102 into aninitial mode of MM waveguide 106, as described above.

According to embodiments of the invention, MDD 112 may include anydemultiplexing device adapted to separate at least one mode of a MMsignal propagating through a MM waveguide into at least one SMwaveguide, respectively.

According to embodiments of the invention MDD 112 may include, forexample, a MDD as described in the International Application titled“Method and Apparatus for Optical Mode Division Multiplexing andDemultiplexing”, filed under the Patent Cooperation Treaty (PCT)concurrently with the present application, assigned to the assignee ofthe present application, the disclosure of which is incorporated hereinby reference in its entirety.

Reference is also made to FIG. 2A, which schematically illustrates a MMdirectional coupler 230 that may be used in conjunction with exemplaryembodiments of the invention.

According to embodiments of the invention, MM directional coupler 230may include a MM waveguide 234, a SM waveguide 236, and a couplingregion 232. Coupling region 232 may allow coupling a desired mode of aMM signal from MM waveguide 234 to SM waveguide 236. According to theseembodiments, coupling region 232 may support at least three modes,including the desired mode and at least two other modes of the MMsignal. Coupling region 232 may include respective portions of waveguide234 and waveguide 236 located in proximity to one another. In theseproximal portions, an effective refractive index of the SM signal may besubstantially equal to an effective refractive index of one of the modesof the MM signal A coupling efficiency of coupling region 232 may dependon a periodic function, e.g., the square of the sine, of a length of thecoupling region. Thus, there may be at least one optimal coupling lengththat provides high coupling efficiency, while the coupling efficiencyfor different lengths of the coupling region, e.g., longer or shorter,the coupling efficiency may be relatively lower.

Reference is also made to FIG. 2B, which schematically illustrates anadiabatic mode demultiplexer 240 that may be used in conjunction withexemplary embodiments of the invention.

According to embodiments of the invention, adiabatic mode demultiplexer240 may include a coupling region 242. Coupling region 242 may includeportions of a MM waveguide 244 and a SM waveguide 246. According tothese embodiments, adiabatic mode demultiplexing may be achieved byadiabatically, e.g., gradually, varying the effective indexes of themodes of waveguides 244 and 246 along coupling region 242, e.g., byappropriately designing the geometric dimensions of waveguides 244and/or 246. Region 242 may provide adiabatic demultiplexing, such that acoupling efficiency of coupling region 242 may be incrementallydependent on a length of the coupling region, e.g., a longer couplingregion may provide a higher coupling efficiency. Thus, the couplingefficiency of region 242 may be substantially insensitive to fabricationerrors.

Although the above description of FIGS. 2A and 2B relates to separatingone mode of a MM signal to yield one SM signal, it may be understood bya person skilled in the art, that the devices including components ofthe devices of FIG. 2A and/or FIG. 2B may be implemented for separatingmore than one mode of a MM signal to yield more than one SM signal,respectively. This may be achieved, for example, by cascading aplurality of MM couplers and/or adiabatic demultiplexers, as describedabove, such that each one of the devices may separate one different modeof the MM signal to a respective SM signal.

According to embodiments of the invention, a MDD having a low level ofcross-talk and/or a relatively high sensitivity to mode-order may beimplemented to improve the separation between the modes switched byswitch 100.

According to embodiments of the invention, MC 108 may include any deviceto produce mode conversion as described above, e.g., a splitter-basedMC, as described below, or a long period grating (LPG) based MC, forexample, a mode converter as described in the International Applicationtitled “Method and Apparatus for Optical Mode Conversion”, filed underthe PCT concurrently with the present application, assigned to theassignee of the present application, the disclosure of which isincorporated herein by reference in its entirety. MC 108 may alsoinclude an adiabatic interferometer-based MC, for example, as describedin the International Application titled “Method and apparatus forinfluencing a light signal”, filed under the PCT concurrently with thepresent application, assigned to the assignee of the presentapplication, the disclosure of which is incorporated herein by referencein its entirety.

Reference is now made to FIG. 2C, which schematically illustrates a LPGmode converter 200 that may be used in conjunction with exemplaryembodiments of the invention.

According to exemplary embodiments of the invention, LPG based MC 200may include an input section 202 associated with MM waveguide 106 (FIG.1), a dynamic waveguide section 204, an output section 206 associatedwith MM waveguide 110 (FIG. 1) and a plurality of control elements 208,for example, a plurality of heating elements that, when activated, areable to change the refractive index of respective regions of dynamicsection 204. Input section 202 and output section 206 may each includean adiabatically shaped, e.g., tapered, waveguide. Control elements 208may be arranged in a configuration that, when activated, is able tocreate a periodic refractive index perturbation pattern along dynamicwaveguide section 204. This perturbation pattern may enable conversionof a propagating light signal between two guided modes.

Reference is now made to FIG. 2D, which schematically illustrates asplitter-based mode converter 250 that may be used in conjunction withsome exemplary embodiments of the invention.

According to embodiments of the invention, MC 250 may include an inputMM waveguide section 252, a beam splitter 254, e.g. a 3 dB splitter, aconversion section 256, a beam combiner 264, e.g., a 3 dB combiner, andan output MM waveguide section 266.

According to embodiments of the invention, an input MM signal enteringMM waveguide 252 may be split by beam-splitter 254 into a firstcomponent and a second component propagating through a first waveguide258 and a substantially parallel second waveguide 259, respectively. Afirst controllable phase shifter, e.g., a heating element 260,associated with waveguide 258, and a second controllable phase shifter,e.g. a heating element 262, associated with waveguide 259, may becontrollably activated to create a phase difference between the firstcomponent and the second component. Subsequently, the first and secondcomponents may be combined, by combiner 264, into a MM output signalexiting waveguide 266. By controllably activating phase shifters 260 and262, controllable mode conversion of at least a fraction of a firstmode-order of the input MM signal into a second mode-order of the outputsignal may be achieved.

Reference is now made to FIG. 3, which schematically illustrates aplanar structure of a M×N optical switch 300 in accordance withexemplary embodiments of the present invention, wherein M and N are bothgreater than one, and to

FIG. 4, which schematically illustrates a cascade 400 of sub-MCs 404, inaccordance with exemplary embodiments of the present invention.

According to embodiments of the invention, M×N optical switch 300 mayinclude M SM input waveguides 302, a passive Mode Division Multiplexer(MDM) 304, a first MM waveguide 306 supporting at least M modes, adynamic MC 308, a second MM waveguide 310, and a passive MDD 312including N SM output waveguides 314.

According to embodiments of the invention, passive MDM 304 may include,for example, a MDM as described in the International Application titled“Method and Apparatus for Optical Mode Division Multiplexing andDemultiplexing”, filed under the PCT concurrently with the presentapplication, assigned to the assignee with of the present application.

According to embodiments of the invention, MDM 304 may include anysuitable device that is able to join a plurality of input signals, froma plurality, M, of SM input waveguides 302, into an M-mode signalcarried by MM waveguide 306, wherein each order mode carries a signalassociated with a particular one of SM waveguides 302. For example, itwill be appreciated by persons skilled in the art that either of thedevices of FIGS. 2A or 2B may be adapted, by reversing the direction ofoperation described above, to perform the desired function of MDM 304,wherein the input signal may include a SM signal and the output signalmay include a MM signal.

According to embodiments of the invention, dynamic MC 308 may includeany suitable dynamic MC, for example, a splitter-based MC, an adiabaticinterferometer based MC, or a LPG MC, as described above. MC 308 may beused to controllably convert a signal carried by at least one of theM-modes of MM waveguide 306 into any mode of MM waveguide 310.

According to exemplary embodiments of the invention, MC 308 may includea cascade 400 of sub MCs 404, for example, Q sub MCs 404, wherein Q isless than or equal to N, and wherein each sub MC is capable ofconverting at least between one pair of different modes.

Passive MDD 312 may include any MDD as described above, and may be usedto separate each mode of order j of MM waveguide 310 into a signalcarried by a corresponding one of SM output waveguides 314.

According to embodiments of the invention, optical switch 300 may haveat least two states of operation, e.g., at least one “On” state ofoperation and an “Off” state, which may correspond, for example, to thestates of operation of MC 308. When MC 308 is at the “off” state, eachone of input waveguides 302 may be associated with a respective one ofoutput waveguides 314, e.g., input waveguide 316 may be associated withoutput waveguide 317, and input waveguide 315 may be associated withoutput waveguide 318.

When the MC is switched to the “On” states, MC 308 may convert at leastsome of the M modes of MM waveguide 306 into different-order modes, forexample, such that MDD 310 may separate the MM signal into a plurality,N, of SM output signals, which may be carried by respective waveguides314 in accordance with any desired scheme. According to exemplaryembodiments of the invention, cascade 400 may be used to sequentiallyconvert M modes into N modes to enable a desired M×N switching effectThus, MC 308 may allow to switch between any desired input waveguide ofM input waveguides 302 and any desired output waveguide of N outputwaveguides 314, e.g., input waveguide 315 may be associated with outputwaveguide 319, and input waveguide 316 may be associated with outputwaveguide 318.

According to some embodiments of the invention, switch 300 may beimplemented as a Variable Optical Attenuator (VOA). This may be achievedby using only one input waveguide, for example one of input waveguides302, and one output waveguide, for example, one of output waveguides314, and operating the switch as described below.

According to some of these embodiments, switch 300 may be implemented asa “normally bright” VOA, wherein the switch transmits light signals fromthe input waveguide to the output waveguide substantially un-attenuatedwhen at the “off” sale. This may be achieved by using the outputwaveguide, which may be associated with the input waveguide, when theswitch is at the “off” state. Thus, when the switch is at the “off”state, no attenuation occurs. When the switch is switched to the “on”states, a controllable attenuation of the light signal may be achieved,by controlling MC 308, as described above.

According to other embodiments, switch 300 may be implemented as a“normally dark” VOA, wherein the switch substantially blocks the lightsignal from being transmitted from the input waveguide to the outputwaveguide when at the “off” state. This may be achieved by using theoutput waveguide, which may not be associated with the input waveguidewhen the switch is at the “off” state. Thus when the switch is at the“off” state, substantial full attenuation occurs. When the switch is atthe “on” states, a controlled attenuation of the light signal may beachieved, by controlling MC 308.

According to exemplary embodiments of the invention, switch 300 mayinclude a combination of MDM, MDD and MC components, as described above,each component having attributes according to a required configurationof the switch. According to these exemplary embodiments, two differentMDDs and/or two different MDMs, for example, each having differentsignal separating attributes, may be used to configure two differentswitches, respectively, as described below.

FIGS. 5 and 6 schematically illustrate a “normally bar” 500configuration and a “normally cross” 600 configuration of a 2×2 opticalswitch, respectively, in accordance with further exemplary embodimentsof the invention.

According to further exemplary embodiments of the invention, switch 500may include a MDM 505 having two input ports, 501 and 502, respectively.An output of MDM 505 may be associated with and input of a MC 506, whichmay include, for example, any one of the mode converters describedherein. An output of MC 506 may be associated with an input of a MDD 507having two output ports, 503 and 504, respectively.

According to exemplary embodiments of tile invention, switch 600 mayinclude a MDM 605 having input ports 601 and 602. An output of MDM 605may be associated with an input of a MC 606. An output of MC 606 may beassociated with an input of a MDD 607 having two output ports 603 and604.

According to exemplary embodiments, MC's 506 and 606 of switches 500 and600, respectively, may operate between two optical modes, e.g., afundamental mode and a first-order mode.

According to an exemplary embodiment of the invention, in “normally bar”configuration 500, the 2×2 switch may allow connection of input ports501 and 502 with output ports 503 and 504, respectively. In the“normally cross” configuration 600, the 2×2 switch may allow switchingbetween the ports, i.e., connecting input ports 601 and 602 with outputports 604 and 603, respectively.

In “normal bar” configuration 500, the MDM may be adapted to associateinput ports 501 and 502 with the fundamental and first-order modes,respectively, of MC 506. The fundamental and first-order modes of MC 506may correspond to a fundamental mode and a first-order mode,respectively, of MDD 507. The fundamental and first-order modes of MDD507, in turn, may be associated with output ports 503 and 504,respectively.

In “normal cross” configuration 600, the MDM may be adapted to associateinput ports 601 and 602 with the fundamental and first-order modes,respectively, of MC 606. The fundamental and first-order modes of MC 606may correspond to a fundamental mode and a first-order mode,respectively, of MDD 607. The fundamental and first-order modes of MDD607, in turn, may be associated with output ports 604 and 603,respectively.

FIG. 7 schematically illustrates another 2×2 optical switch 700, inaccordance with exemplary embodiments of the invention. Optical switch700 may include two SM input waveguides, 701 and 703, a Y couplingbranch structure 702, a first MM waveguide 704, a MD 706, two SM MDwaveguides, 708 and 709, two controllable phase-shifters, 710 and 711,respectively, a MDM 712, a second MM waveguide 714, a splitting Y-branchstructure 716, and two SM output waveguides, 718 and 719. According toembodiments of the invention, switch 700 may be operated, independentlyof passive MDD 706 and/or MDM 712 characteristics, in a “normally bar”configuration and/or a “normally cross” configuration, as describedabove.

According to embodiments of the invention, an input signal entering oneof the input waveguides, e.g., waveguide 701, may be combined bystructure 702 to an initial MM signal, propagating through MM waveguide704, and including two mode orders, e.g., a zero-order mode and afirst-order mode. MDD 706 may be used to drop the first-order mode andto enable two SM signals, each having substantially half of the power ofthe input signal, to exit MDD 706 and enter SM waveguides 708 and 709,respectively. Phase shifters 710 and 711 may include tunable controlelements, e.g., heating elements that affect the propagationcoefficients of the propagating signals, which may be controllablyactivated to create a predetermined phase difference between the twosignals, for example, by differential heating. MDM may combine the twoSM signals into one converted MM signal propagating through MM waveguide714. Structure 716 may be used to direct the converted MM signal to oneof waveguides 718 and 719, depending on the phase shift created betweenshifters 710 and 711.

According to exemplary embodiments of the invention, a zero phasedifference between the signals may yield a “cross” state of switch 700such that, for example, a signal entering waveguide 701 may exit theswitch through waveguide 718. A phase shift of π (pi) radians betweenthe signals may yield a “bar” state of switch 700 such that, forexample, a signal entering waveguide 701 may exit the switch throughwaveguide 719

According to some embodiments of the invention, switch 700 may beimplemented as a VOA. This may be achieved by using only one input ofthe switch, e.g., input 701, and creating a predetermined phase shift ofmore than zero and less than π radians, e.g., by controllably activatingshifters 710 and 711, respectively, to tune the amount of phase-shiftingapplied to the signals. It will be appreciated by persons skilled in theart that tunable activation of shifters 710 may allow controlling anattenuation level of a signal entering input 701, and may also allow“normally dark” and/or “normally bright” attenuation, as describedabove.

FIG. 8 schematically illustrates a block-diagram of a method 800 ofswitching M input signals into N output signals, in accordance withexemplary embodiments of the invention.

Method 800 may begin with joining M input signals into a first MM signalhaving M initial modes, e.g. using a MDM and/or a beam combiner, asindicated at block 801.

The first MM signal may be converted, e.g., using a MC, into a second.MM signal having N converted modes, which may be different from theinitial modes, as indicated at block 802.

Finally, the second MM signal may be separated, e.g., using a MDD and/ora beam splitter, into N SM output signals, as indicated at block 803.

According to embodiments of the invention, the converting of the firstsignal into the second signal may include dynamically controlling arelationship between the converted modes and the initial modes such thateach one of the M input signals may be switched to one of the N outputsignals according to a desired scheme.

According to embodiments of the invention, the switches described abovemay use the difference between the initial mode-orders and the convertedmode-orders to provide various switching functions, for example, thevariety of switching functions described herein. Since each of themode-orders may have significantly different optical properties, e g.,effective refractive index, a relatively high level of separationbetween the output ports of the switch may be achieved, e.g., a highratio between the power of a signal exiting an intended output of theswitch and the power of signals exiting unintended outputs of theswitch.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.Embodiments of the present invention may include other apparatuses forperforming the operations herein. Such apparatuses may integrate theelements discussed, or may comprise alternative components to carry outthe same purpose. It will be appreciated by persons skilled in the artthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

1. An optical switching device for switching M input signals into Noutput signals, wherein M and N are each equal to or greater than one,comprising: a mode division multiplexer to join said M input signalsinto a first multi-mode signal having M initial modes; a mode converterto convert said first multi-mode signal into a second multi-mode signalhaving N converted modes; and a mode division demultiplexer to separatesaid second multi-mode signal into said N output signals, wherein saidconverter is able to be controllably activated such that said Nconverted modes are separated by said demultiplexer to said N outputsignals according to a desired scheme.
 2. The device of claim 1 whereinsaid mode converter comprises a dynamic, controllable, mode converterable to convert at least a fraction of at least one of said M initialmodes into a respective one of said N converted modes.
 3. The device ofclaim 1 wherein said mode converter comprises a long period grating modeconverter.
 4. The device of claim 1 wherein said mode convertercomprises a splitter-based mode converter.
 5. The device of claim 4wherein said splitter-based mode converter comprises: a multi-mode inputwaveguide; a beam splitter associated with said multi-mode inputwaveguide, to split said first multi-mode signal into a first componentpropagating through a first single mode waveguide and second componentpropagating through a second, substantially parallel, single modewaveguide; first and second controllable phase shifters associated withsaid first and second single mode waveguides, respectively; and acombiner to combine said single mode signals into an output multi modewaveguide, wherein said first and second phase shifters are able to becontrollably activated to create a desired phase shift between saidfirst and second components to convert at least a predetermined fractionof a first mode-order of said input signal into a second mode-order ofsaid output signal.
 6. The device claim 1 wherein said mode convertercomprises a cascaded set of sub-converters, each sub-converter adaptedto convert at least one mode of said M initial modes into acorresponding mode of said N converted modes.
 7. The device of claim 1further comprising a multi-mode waveguide to connect said mode divisionmultiplexer to said mode converter.
 8. The device of claim 1 furthercomprising a multi-mode waveguide to connect said mode converter to saidmode division demultiplexer.
 9. The device of claim 1 wherein M is equalto one, and wherein said mode division multiplexer comprises a waveguideexpander to adiabatically expand a single mode waveguide input of thewaveguide expander into a multi-mode waveguide output of the waveguideexpander.
 10. The device of claim 1 wherein both M and N are equal totwo.
 11. The device of claim 10 wherein said mode converter is adaptedto switch between two mode orders of said first multi-mode signal. 12.The device of claim 11 wherein said two mode orders are a zero-ordermode and a first-order mode, respectively.
 13. A variable opticalattenuator comprising the device of claim 1, wherein M is equal to one,and wherein said converter is controllably activated to transmit adesired attenuated fraction of said input signal to a predeterminedoutput signal of said N output signals.
 14. The device of claim 13,wherein said variable optical attenuator is normally dark and whereinsaid attenuated fraction equals substantially zero when said converteris not activated.
 15. The device of claim 13, wherein said variableoptical attenuator is normally bright and wherein said attenuatedfraction equals substantially the full intensity of said input signalwhen said converter is not activated.
 16. An optical 2×2 switchingdevice for switching a single-mode input signal entering one of twosingle-mode input waveguides, into one of two single-mode outputwaveguides, comprising: a Y-coupling branch structure associated withsaid input waveguides to combine said input signal to an initialmulti-mode signal, having two mode orders; a mode division demultiplexerto drop one of said two mode orders and to produce first and secondsingle-mode signals propagating through first and second MO single-modewaveguides, respectively; first and second controllable phase shiftersassociated with said first and second MD single mode waveguides,respectively; a mode division multiplexer to combine said first andsecond single mode signals into a converted multi-mode signal; and asplitting Y-branch structure to direct said converted multi-mode signalto one of said two single-mode output waveguides, wherein said first andsecond phase shifters are able to be controllably activated to create adesired phase shift between said first and second single-mode signals,such that said splitting structure directs said converted multi-modesignal to a desired output waveguide of said two single-mode outputwaveguides.
 17. The switching device of claim 16 wherein said first andsecond controllable phase shifters comprise first and second heatingelements, respectively.
 18. A variable optical attenuator comprising theswitching device of claim 16 wherein said first and second phaseshifters are activated to cause the switching device to transmit adesired attenuated fraction of said input signal to said desired outputwaveguide.
 19. A variable optical attenuator according to claim 18wherein said attenuated fraction is equal to substantially the fullintensity of said input signal one when said phase shifters are notactivated.
 20. A variable optical attenuator according to claim 18wherein said attenuated fraction equals substantially equals zero whensaid phase shifters are not activated.
 21. The variable opticalattenuator of claim 18 wherein said desired phase shift is between 0radians and π radians.
 22. A method of switching M input signals to Noutput signals, wherein M and N are each equal to or greater than one,comprising: joining said M input signals into a first multi-mode signalhaving an initial set of M modes; converting said first multi-modesignal into a second multi-mode signal having a converted set of Nmodes; and separating said second multi-mode signal into said N outputsignals.
 23. The method of claim 22 wherein converting said firstmulti-mode signal into said second multi-mode signal comprisesdynamically controlling a relationship between said converted modes andsaid initial modes, such that each one of said M input signals iscontrollably switched to one of said N output signals.
 24. The method ofclaim 22 wherein converting said first multi-mode signal into saidsecond multi-mode signal comprises activating a long period gratingperturbation pattern
 25. A splitter-based mode converter for convertingan input MM signal into a MM output signal comprising: a multi-modeinput waveguide; a beam splitter, associated with said multi-mode inputwaveguide, to split said input MM signal into a first componentpropagating through a first single mode waveguide and a second componentpropagating through a second, substantially parallel, single modewaveguide, respectively; first and second controllable phase shiftersassociated with said first and second single mode waveguides,respectively; and a combiner to combine said single mode signals into anoutput multi mode waveguide, wherein said first and second phaseshifters are able to be controllably activated to create a desired phaseshift between said first and second components to convert at least afraction of a first mode-order of said input signal into a secondmode-order of said output signal.
 26. A device for coupling a mode of amulti-mode input signal from a multi-mode waveguide to a single-modewaveguide, the device comprising a coupling region associated withmutually proximal regions of said multi-mode waveguide and said singlemode waveguide, respectively, wherein the length of the coupling regionis such that a desired fraction of said mode of the input signal iscoupled to said single-mode waveguide.